A variety of drive systems for AC machines utilizing electronic switching to control the power applied to the machines are presently available commercially. These AC machine drives allow the speed and/or torque of the machine to be controlled to meet various requirements. Such machine drives typically require mechanical shaft transducers to provide feedback of shaft position and/or velocity. Feedback is required both for torque control (i.e., field orientation or vector control) and trajectory tracking. However, shaft transducers and the associated wiring to provide the signals from the shaft transducers to the electronic drive add significantly to the cost and rate of failure of the system, and also add to the total volume and mass of the machine at the work site. Because induction machines are generally lower in cost and more rugged than other machine types, to a large extent the advantages of induction machines are the most compromised by the addition of such transducers.
Consequently, the desirability of eliminating position or velocity transducers in motor motion control applications has long been recognized. Several approaches have been proposed to allow estimation of the rotor position or velocity. Some success, although limited, has been obtained with techniques for determining the rotor position in synchronous and reluctance machines, which are considerably less complex than induction machines and have inherent spatially dependent rotor properties that can be easily tracked. Estimation of rotor position and velocity in the induction machine, which is by far the most common machine type and thus has the most significant commercial potential, is complicated because of its smooth symmetric rotor and symmetric induced rotor currents and slip. Nonetheless, accurate and parameter insensitive position and velocity measurement in induction machines can only be obtained by tracking spatial phenomena within the machine.